Latex based adhesive composition for paperboard and corrugated board

ABSTRACT

This invention relates to an adhesive of starch and an ammonium-based latex which provides paperboard and corrugated board treated therewith with improved wet-pin adhesion. The latexes are typical styrene-butadiene and carboxylated styrene-butadiene latexes but untypically are polymerized in the presence of a volatile base such as ammonium hydroxide.

BACKGROUND OF THE INVENTION

Starch-based adhesives have been extensively employed for variousadhesive applications, especially in the area of paperboard andcorrugated board. Generally, starch-based adhesives for corrugation areformulated from cooked or gelatinized starch and raw starch particles.The cooked starch increases viscosity and serves as a suspending agentfor the raw starch particle. Heating the starch mixture results ingelation of the uncooked starch yielding higher viscosity and improvedbonding characteristics.

A major consideration in the preparation of starch-based adhesives isthe ultimate adhesive strength. One measurement of adhesive strength ispin strength. Pin strength is the force (measured in pounds) required todelaminate a liner board from the medium in a corrrugated boardconstruction.

A further consideration with respect to pin strength is moreparticularly wet-pin strength. Wet-pin strength is the pin strength of acorrugated board construction after having been soaked in water. Wherewet strength is important it is necessary for adhesives to haveexcellent adhesion under high humidity and wet conditions. It is alsoimportant that this excellent water-resistance be achieved under minimalcure conditions. Conventional starch-based adhesives generally have poorwater-resistance, therefore it is desirable to prepare an adhesive forpaperboard and corrugated board having improved water-resistance.

SUMMARY OF THE INVENTION

The present invention is an aqueous adhesive comprising starch and anammonium-based latex. Preferably, the adhesive is prepared in theabsence of a caustic component, such as sodium hydroxide.

Utilizing an ammonium-based latex improves the water-resistance of theadhesive and therefore, improves the wet-pin strength of the corrugatedboard.

DETAILED DESCRIPTION OF THE INVENTION

The aqueous adhesive of the present invention comprises an ammonia-basedlatex and starch.

The starch component of the adhesive is generally present in an amountof from about 20 to about 95 dry parts, based on 100 parts of dryadhesive. More preferably, the starch is present in an amount of fromabout 40 to about 90 dry parts per 100 parts of dry adhesive. Generally1 to 10 parts of the starch component is cooked or gelatinized toprovide for viscosity in the final adhesive. Alternatively, syntheticthickeners can be used to develop the required initial adhesiveviscosity eliminating the need to cook any of the starch.

In the preparation of the example adhesive, a minor portion of thestarch is cooked by first blending with water and heating untildissolved. This primary mixture can then be added to a secondary mixturecontaining the latex and remaining starch component. The two mixturesare then thoroughly mixed to prepare the subject adhesive.

The latex component of the subject adhesive, which is generally added tothe uncooked portion of the composition, is generally present in anamount of from about 80 to about 5 dry parts per 100 parts dry adhesive;more preferably, from about 50 to about 10 dry parts based on 100 partsdry adhesive. From an economic standpoint, it is more desirable tomaintain the latex at as low a level as possible.

By "ammonia-based latex" is meant a polymer latex prepared in thepresence of ammonium hydroxide. Normally a non-volatile base, such assodium hydroxide, is used as a neutralent (to raise pH) duringpolymerization. However, for the latex used in the present latex basedadhesive, a volatile base such as ammonium hydroxide acts as thepolymerization neutralent.

Typical latex compositions which can be employed in the subject latexbased adhesive are those generally known in the art and polymerized inthe presence of a volatile base such as ammonium hydroxide. Typicalexamples of latexes which can be employed in adhesives of the presentinvention are styrene-butadiene latexes and carboxylatedstyrene-butadiene latexes similar to those disclosed in U.S. Pat. No.4,396,453, the difference being the present latexes are ammoniumhydroxide based rather than sodium hydroxide based.

Employing modified styrene-butadiene latexes such as isopropenyloxazoline-modified styrene-butadiene latex in the subject adhesivecomposition yields improved water-resistant characteristics as well. Theoxazoline-modified polymer compositions are generally disclosed in U.S.Pat. No. 4,508,869 and 4,474,923, herein incorporated by reference.These oxazoline-modified polymers are disclosed to have good tensile andelongation properties and excellent resistance to water and aqueousfluids. These properties are enhanced when the latex is polymerized inthe presence of ammonium hydroxide.

The functional monomer, isopropenyl oxazoline, is typically present inthe latex, in an amount from about 0.25 to about 10 parts per 100 partsby weight polymer latex, more preferably from about 1 to about 2 partsper 100 parts polymer latex.

In the preparation and testing of various latex based adhesiveformulations employing the starch and the ammonium-based latex, it hasbeen found that by minimizing the amount of caustic component, such assodium hydroxide, employed in the preparation of the latex basedadhesive that wet-pin strength is substantially improved. Thus, thepreferred latex based adhesive of the present invention comprises only aminor amount of caustic component, more preferably no caustic component.

The latex based adhesive composition of the present invention can beutilized in the production of corrugated paperboard and corrugatedpaperboard products such as containers. The corrugated paperboardtypically comprises a fluted medium and a liner adhesively joined to thetips of the fluted medium on one or both sides thereof. (The adhesivemade from the present latex is not limited to the above-describedcorrugated paperboard but can be used for other corrugated boardconstruction as well.) The adhesive composition of the present inventioncan be applied to the fluted medium or the liner, however it is usuallypreferred to apply the adhesive to the tips of the fluted medium.

The adhesive can be employed in amounts from about 1 to about 10 poundsper 1,000 square feet based upon the dry weight or solids weight of theadhesive. Preferably, the adhesive is employed in amounts of from about1 to about 5 pounds per 1,000 square feet.

Any of various paperboard substrates can be utilized in combination withthe present adhesive composition in order to provide corrugatedpaperboard having improved wet-pin strength. The latex based adhesive ofthe present invention also provides water-resistant properties. Thepresent composition can be employed in a variety of paperboard productsincluding wax or resin impregnated paperboards, which would furtherimprove the water-resistant characteristic of a corrguated paperboardproduct prepared with the present invention.

EXAMPLE 1

Into a one-gallon, jacketed reactor equipped with lab pumps to delivermonomer and aqueous feeds are added 1088.5 g of deionized water, 14.77 gof a one-percent active aqueous pentasodium diethylene triaminepentacetate solution, 29.53 g of itaconic acid and 26.18 g of a 39.7percent solids seed latex containing polystyrene particles having avolume average particle size of about 270 Å. The reactor is purged withnitrogen and heated to 90° C. Then, over a two hour and 40 minutesperiod is added a monomer stream containing 590.63 g of styrene and 5.17g of tertiary dodecyl mercaptan.

Beginning simultaneously with the start of the monomer stream is added asecond monomer stream, also over a two hour and 40 minute period, 355.0g of butadiene.

Beginning simultaneously with the start of each of the above monomerstreams is added continuously over a five hour period, 295.3 g ofdeionized water, 16.41 g of a 45 percent active aqueous surfactantsolution, 14.77 g of a 10 percent aqueous ammonium hydroxide solution,and 10.34 g of sodium persulfate. After three hours and for five minutesis added 78.55 g of a 10 percent ammonium hydroxide solution. Afterthree hours and 10 minutes for one hour and 20 minutes is added anothermonomer stream containing 2.21 g of tertiary dodecyl mercaptan, 14.77 gof isopropylene oxazoline, 302.7 g of styrene and 184.0 g of butadiene.Following the addition of the monomer streams and aqueous streams, thereaction mixture is maintained at 90° C. for one additional hour andthen cooled.

ADHESIVE COMPOUND PREPARATION

A mixer is charged with 400 pounds of cold water. Forty-five pounds ofcorn starch are added and the mixture is agitated until the corn starchis dispersed. The mixture is then heated to 85° C. and held at 85° C.for 15 minutes with mixing to gelatinize the starch. Fifty pounds ofcold water are then added to cool the mixture.

In a separate mixer, 2,000 pounds of 50 percent solids latex preparedsimilarly to the latex in example 1, are added. In another smallercontainer, 10 pounds of 10 mol. borax are dissolved with agitation in 70pounds of water, heated to 60° C. The solubilized borax/water mixture isadded, with agitation, to the latex in the mixer. Then 665 pounds ofcorn starch are added and the mixture is agitated until all of the cornstarch is well dispersed. Thereafter, the mixture of 45 pounds ofgelatinized corn starch in 450 pounds of hot water prepared above isadded slowly. Mixing is continued until a smooth, homogeneous adhesiveis obtained. The resultant adhesive has a viscosity of 2,700 cps, asmeasured with a Model RVT Brookfield Viscometer at 20 RPMS.

EXAMPLE 2

Into a one-gallon, jacketed reactor equipped with lab pumps to delivermonomer and aqueous feed are added 1160.67 g of deionized water, 0.37 gof a 40 percent active aqueous pentasodium diethylene triaminepentacetate solution, 29.69 g of itaconic acid and 26.12 g of a 40percent solids seed latex containing polystyrene particles having avolume average particle size of about 270 Å. The reactor is purged withnitrogen and heated to 90° C. Then, over a four hour period is added amonomer stream containing 556.60 g of butadiene. Beginningsimultaneously with the start of the butadiene monomer stream is addedcontinuously over a four hour and 30 minute period, a monomer streamcontaining 897.98 g of styrene and 7.42 g of tertiary dodecyl mercaptan.Also, beginning simultaneously with the start of the monomer streams isadded continously over a four hour and 30 minute period, 296.85 g ofdeionized water, 30.83 g of a 33.7 percent solution of sodiumpersulfate, 5.30 g of a 28 percent solution of ammonium hydroxide and15.49 g of a 45 percent active aqueous surfactant solution. Followingaddition of the monomer and aqueous streams, the reaction mixture ismaintained at 90° C. for one additional hour and then cooled.

The latex/starch adhesive is then prepared as described above. Theboxboard is then tested for wet-pin adhesion by soaking a 2×4 inch stripof boxboard in water for one hour, mounting the boxboard on an Instronmachine and recording the force required to reach delamination.

The wet-pin adhesion, as is shown in the following table, improves whenthe latex in the adhesive is ammonium hydroxide based rather than sodiumhydroxide based. (Comparatie Example 2 is prepared similarly to Example2 but with sodium hydroxide and not ammonium hydroxide.)

    ______________________________________                                        POLYMERIZATION NEUTRALANT                                                     VS                                                                            WET-PIN ADHESION                                                                            Neutralant Used in                                                                           Wet-Pin                                          Adhesive      Latex Recipe  (lbs.)                                            ______________________________________                                        Example 1     NH.sub.4 OH   65                                                (IPO                                                                          modified)                                                                     Example 2     NH.sub.4 OH   60                                                Comparative   NaOH          15                                                Example 2                                                                     ______________________________________                                    

What is claimed is:
 1. An aqueous adhesive comprising:(a) starch; and(b) a polymer latex prepared in the presence of ammonium hydroxide. 2.The adhesive of claim 1 wherein said latex is anisopropenyl-2-oxazoline-modified latex.
 3. The adhesive of claim 1wherein said starch is present in an amount of from about 20 to about 95dry parts per hundred parts dry adhesive and said ammonium-based latexis present in an amount of from about 80 to about 5 dry parts perhundred parts dry adhesive.
 4. The adhesive of claim 1 wherein theammonium-based latex is selected from the group consisting ofstyrene-butadiene latex or carboxylated styrene-butadiene latex.
 5. Theadhesive of claim 1 wherein said latex is anisopropenyl-2-oxazoline-modified styrene-butadiene latex.
 6. Theadhesive of claim 5 wherein said isopropenyl-2-oxazoline is present inan amount of from about 0.25 to about 10 parts by weight based on 100parts polymer latex.
 7. The adhesive of claim 6 wherein saidisopropenyl-2-oxazoline is present in an amount of from about 1 to about2 parts by weight based on 100 parts polymer latex.
 8. An aqueousadhesive consisting essentially of:(a) starch; and (b) a polymer latexprepared in the presence of ammonium hydroxide.